The fuel system of an internal combustion engine generally includes a pressure reducing valve that leaks fuel to reduce the fuel pressure in an accumulator for accumulating fuel at high pressure. Apparatuses for diagnosing the presence/absence of an abnormality in the leak function of a pressure reducing valve have been proposed (for example, see Patent Document 1).
In Patent Document 1, a diagnosis is performed of a pressure reducing valve serving as a leakage mechanism and of a fuel pressure sensor for detecting the fuel pressure in the accumulator. Among these, in particular, for the diagnosis of the fuel pressure sensor, the time at which the fuel pressure sensor is diagnosed is set in consideration of the fuel viscosity, which varies in accordance with fuel temperature changes. That is, the time required for the pressure inside of the accumulator to decrease to atmospheric state from a high-pressure state by opening the pressure reducing valve is extended as the fuel viscosity is increased even if the pressure reducing valve is normal. The fuel viscosity increases as fuel temperature is lowered. Therefore, in Patent Document 1, the time from when the pressure reducing valve is opened to when diagnosis is performed is set longer as the fuel temperature is lower to ensure diagnosis accuracy in diagnosing the presence/absence of an abnormality in the fuel pressure sensor.